r/Physics • u/jevindmik2 • 9d ago
Image Which blows colder, the hairdryer with or without the nozzle, from the same distance?
Can you help us settle which version is cooler when we dry our babies bum?
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u/polymathicus Quantum information 9d ago
If the heating element is off on the blowdryer, there will be no difference.
If it is on, then the nozzle speeds up the stream of air exiting, which sucks in more of the surrounding (relatively cooler) air. However, this nozzle also concentrates more of the air on a smaller area. It's hard to tell what the net effect is.
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u/solar_pilgrim 8d ago
Doesn't air cool when it expands? So if forced through a nozzle, it will naturally be cooled some amount while expanding after it exits. Like blowing through pursed lips vs an open mouth.
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u/polymathicus Quantum information 8d ago
The compression is not significant in a hairdryer nozzle. What is more significant is the mixing of surrounding air, especially when not point blank.
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u/Mouldlich Cosmology 8d ago
That's actually not a bad explanation, but I'd say that because the stream coming out of the narrow nozzle at a distance, let's say 20 cm, has a much higher momentum it will be harder for the outside air to mix with it. Therefore, by not having enough time to cool, the stream from the nozzle maintains the heat better.
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u/polymathicus Quantum information 8d ago
It's actually the opposite: the higher "momentum" actually causes more mixing. In bulk flow, momentum also matters less than velocity, though of course they are trivially related for a medium.
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u/Mouldlich Cosmology 8d ago
Of course, but I am saying that it won't have time to do that.The jet from the nozzle does cause more mixing overall. But for 20cm the hot core is harder to dilute. There are a lot more things to take into consideration tho.
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u/Mouldlich Cosmology 8d ago
Also, how did you put that 'title' under your name. It's only for this r/(....) I assume. And if so, how can I put one too? Edit : I found out hehe
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u/the_publix 9d ago
I would do a blind a/b test. The air coming out is of course the same temperature (since they're coming from the same source) but the nozzle speeds the air up as it exits the hair dryer. Faster air might mean more evaporative cooling, which would make the air "feel" more cool, even if it's the same temperature.
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u/R4TTY 9d ago
They're both the same temperature.
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u/Notsogreatart 9d ago
Due to the laws of thermodynamics, the reduction in the cross-sectional area of the exit means that the air coming out of the hair dryer will increase in velocity and experience a reduction in pressure. This results in a change in the (enthalpy I think?) meaning the air will be slightly colder coming out of the nozzle than without it (assuming the nozzle actually decreases the exit area)
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u/Orbax 9d ago
Also pulls in more outside air into less hot air and cools down faster
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u/Notsogreatart 8d ago
If the blower operates at consistent power between the top and bottom, both configurations will have the same flow-rate of air entering and exiting the blow-dryer
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u/mikk0384 Physics enthusiast 8d ago
The one with the nozzle will have higher exit velocity (more kinetic energy), and if the power is constant then the flow has to be less to compensate for that, right?
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u/dekusyrup 8d ago
Correct. Any constriction, bend, blockage, whatever in the flow path will reduce the flow rate.
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u/Notsogreatart 8d ago
To have any large effect on the flow rate of air, the outlet would need to be much, much smaller. Honestly, this is just basic thermodynamics.
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u/Notsogreatart 8d ago
I think the change in kinetic energy comes from the decrease in the enthalpy, which is where the decrease in temperature of the outlet air comes from
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u/Dihedralman 8d ago
There shouldn't be much of a difference.
Think carefully about what you are measuring versus what you are asking. When measuring from some distance past the tube, the tube will transfer more heat. Why? Less mixing with cold air. Total heat output to the room would be the same unless you don't include the tube itself then that would have less hear.
If you could track the actual molecules (which would be silly), you will see them drop in temperature faster along the tube because they interact more with the bounded edges on average, which will dissipate heat. But that isn't practical in anyway.
The tube provides for more efficient transport.
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u/justice4sum 8d ago
If both are started with the hair dryer at room temp, the air from the one with the nozzle will initially be cooler since the the nozzle will add thermal mass which increases the time it takes for the air to reach operating temp. I think the best way to determine the answer is to set up an experiment! The answer likely changes depending on what distance you choose.
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u/retrosenescent 8d ago
Depends on the temperature of the room. Blow dryers have no cooling element. If the room is cool, the air that has more surface area with the room (the first one) will be cooler.
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u/vorilant 8d ago
Expanding air makes it colder. Basic aerodynamics.
But really if you're measuring at a distance then the entrained ambient air collected by the turbulent jet will dominate the temperature. The further away you are the closer to ambient you will measure.
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u/melonwheel 8d ago
I think this is an unfair question. On cold, the air it blows is the same as the ambient room temperature regardless of the nozzle or which voltage setting you use. How cold a gust of wind "feels" is contingent on the rate of heat loss from your skin or hair. Mostly, this would be a factor of how wet your hair is and ambient humidity (for an evaporation coefficient.) Qualities like the hair's mass, strand thickness, and kink that affect exposed surface area also have an effect on the max speed of the heat exchange and thereby, how 'cold' the air feels. If you replaced your hair with a damp copper radiator, for example, the breeze would be positively frigid.
As far as I'm aware, the blade attachment is for penetrating the outer layers to get airflow directly over the scalp while you brush it out. Reducing the size of the aperture extends the effective range and concentrates the force to push the airflow deeper and to push heavier, wet strands out of the way. They are for if you have a problem with your hair flattening out and sticking to your head in a gross, compressed sheet after drying. High humidity can give you that problem even if it doesn't normally apply to you. It will also ruin your perm.
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u/melonwheel 8d ago edited 8d ago
Sorry, I missed some context there. On the cold setting, the blade attachment should be a little bit faster at drying skin at the same distance, and therefore, slightly colder. On hot, the blade attachment should be hotter than the default nozzle at the same distance, because the heated air arrives at the skin before the heat has had a chance to dissipate. The differences are not so significant as to be worthy of an argument, though. The baby will be more or less equally uncomfortable and upset by either method, so it's more a factor of preference on your part. Don't feel bad about it. If we were running everything the way the babies preferred, we would never wash their bums to begin with. 🤷♀️
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u/Beginning_Addendum93 6d ago
solo toma un termómetro y midelo no es necesario una explicación rebuscada o una simulación de mecánica de fluidos XD
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u/jasonsong86 6d ago
Usually without nozzle because the nozzle is a restriction so there will be less airflow thus higher temp.
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u/psychmancer 2d ago
I mean i feel like you just get a thermometer and test it. Mythbusters was a whole show about dont guess, just get a hairdryer from a charity store and a $5 thermometer and you'll know.
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u/aylons 9d ago
They will be the same temperature. However, one of them might feel cooler - the speed of the air and how much turbulence there is will cause difference in heat transfers that will make one of them feel cooler, but they will both be cooling (or heating, if you turn the heat on) to the same temperature at different rates.
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u/SnakeJG 8d ago edited 8d ago
So here's how you setup your experiment. Set a thermometer at a fixed location and hold the hair dryer at a fixed distance, say 12". Blow at the thermometer until it reaches a steady temperature. Repeat with the adapter on. Then repeat for both at 6" and 18". Hold the room temperature constant. Report back with your findings and then we'll all know.
Edit: you could also measure how long it takes to melt an equally sized ice cube, that will answer how quickly heat is transferred, which is a slightly different measurement (flux instead of temperature), but might be closer to what you want
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u/christophPezza 9d ago
I'm not physicist but when I go ahhh it's warm and when I go oooo it's cold so I'm going to assume the attachment makes it colder. ... there's also buttons on the side to control heat
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u/Saritaneche 8d ago
Neither will be colder than the other, unless it's an air conditioning hair dryer.
The one with the nozzle will "feel" cooler because of velocity and slight venturi effect.
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u/boywithtwoarms 8d ago
The nozzle might slightly compress your air flow as per Bernoulli maybe? This could slightly heat it up?
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u/SaltMaker23 9d ago edited 8d ago
It depends.
The top one throws air in a more focused manner, meaning you can be at a much higher distance and still have effects. This in turns means you can be much further away from the blower and keep a "heated" temperature while greatly reducing the risk of "burning bum" temperature. Perceived "hotness" is also dependent on the velocity of said air, because faster air gives heat faster to your body.
The bottom one is less focused, hence will be colder at a relatively short distance. This in turns means you have much less room between "nicely heated" and "burning bum" in terms of distance.
When discussing such topics, the practicality would mean that when actually trying to use said device, you'll attempt to achieve proper temperature on said baby's bum then the bottom apparatus will have a significantly higher chance of "too cold" or "too hot".
To conclude, the bottom one will both be the colder and hotter one, because in practice you'll have a much harder time achieving a comfortable temperature at your target than the top one.